Electrolytic system for production of alkali metals



United States Patent [1.8. Cl. 204-68 8 Claims ABSTRACT OF THE DISCLOSURE A process for recovering an alkali metal from its molten amalgam by electrolysis of the amalgam in an electrolytic cell utilizing an alkali metal amide-containing molten electrolyte fusible below 180 C. The alkali metalamalgam cell may also be utilized as a subcell of a unitary electrolytic cell for producing an alkali metal starting with electrolysis of an aqueous solution of an alkali-metal salt. Exemplary and preferred is the recovery of sodium from its amalgam in an electrolytic cell wherein the molten electrolyte is fusible below 160 C. and is a ternary salt mixture of sodium amide, sodium hydroxide, and sodium iodide, the amalgam being produced preferably by electrolysis of an aqueous sodium chloride solution using a mercury or amalgam cathode.

Cross references to related applications Various sodium and potassium salt mixtures that may be utilized as molten electrolyte in conjunction with various embodiments of this invention relating to the recovery of sodium and potassium from their molten amalgams are described in Fusible Alkali-Metal Salt Electrolyte, filed of even date herewith and assigned to the assignee of the present invention. Reference should be made to this application for fuller details of the sodium and potassium salt electrolytes that may be utilized in the practice of the present invention.

Background of the invention This invention relates to an improved electrolytic process for recovery of an alkali metal from its amalgam. More particularly, the present invention relates to a low-temperature electrolytic process for the recovery of sodium, potassium, and mixtures thereof from their amalgams.

Of the alkali metals, sodium in particular is finding increasing use because of its desirable physical, chemical, and electrical properties. However, its more widespread utilization has been limited because of the cost involved in producing it by means of existing processes. In the Castner process for the production of metallic sodium by electrolysis of fused sodium hydroxide, the current efiiciency is generally less than 40 percent, rendering the process uneconomical. In the Downs process in which fused sodium chloride is electrolyzed at a temperature above 800 C., the high temperature of operation presents many disadvantages.

To overcome the disadvantages of the Castner and Downs processes, the Gilbert process, shown in US. 2,148,404, proposes producing an alkali metal amalgam by electrolysis of an equeous solution of an alkali-metal salt in the presence of a mercury (or amalgam) cathode, and then recovering the alkali metal electrolytically from the so-produced amalgam by using an anhydrous fused mixture of alkali metal hydroxide and alkali metal halide as electrolyte. For the production of sodium from sodium amalgam, Gilbert proposes as molten electrolyte a sodium hydroxide-sodium iodide eutectic (M.P. 225 C.) at a cell operating temperature of 230-250 C. At

3,472,746 Patented Oct. 14, 1969 the temperature of operation of the molten salt electrolyte, the mercury vapor pressure is about mm., resulting both in significant loss of mercury as well as is contamination of the produced sodium by mercury.

In German Patent 862,519, a molten salt electrolyte that isa ternary eutectic of NaOH-NaI-NaBr is proposed for electrolytic recovery of sodium from a sodium amalgam. This electrolyte has a melting point of 215 C. and serves to lower the operating temperature of the electrolytic cell by about 10 degrees compared with Gilberts process. In US. Patent 3,265,490 a ternary mixed salt consisting of NaOH-NaI-NaCN (M.P. 195 C.) is proposed as molten electrolyte in a sodium amalgam electrolytic cell for operation at a temperature of 210- 230 C. While the two ternary mixtures described above result in operation at a slightly lower temperature, the mercury loss and contamination of the produced sodium by mercury are still significant. Furthermore, cell operation above 200 C. requires use of expensive corrosionresistant materials.

Story, in US. 3,006,824, shows an electrolytic cell in which a thin, vertical supported film of mercury is used as an interelectrode. To provide suitable operating temperature, he proposes as nonaqueous electrolyte a solution of sodium iodide in diethylene glycol dimethyl ether. However, this solution, and, generally, solutions of sodium salts in organic solvents have much lower electrical conductivity than molten sodium salts. This results in a very high internal resistance for the cell operation, making the use of such electrolytes economically impractical.

Summary of the invention It is an object of this invention to provide an electrolytic process for the recovery of an alkali metal from its molten amalgam at a temperature lower than heretofore feasible, thereby reducing loss of mercury and further providing a liquid alkali metal of enhanced purity. The lower operating temperature permits use of inexpensive plastics as cell construction materials.

It is a further object to provide a process for the production of an alkali metal from an aqueous solution of its chloride in a unitary electrolytic cell wherein the entire cell is maintained and operated at essentially the same temperature.

In accordance with this invention, an electrolytic process is provided for the recovery of an alkali metal selected from sodium, potassium, and mixtures thereof from its molten amalgam by using in the electrolytic cell as molten electrolyte a fused salt mixture melting below 180 C. and containing as essential component the amide of the alkali metal. The process is particularly suitable for the recovery of sodium from sodium amalgam using as molten electrolyte fusible below C. a ternary mixed salt having a composition, in mole percent, of 30-78 sodium amide, 20-60 sodium hydroxide, and l20 sodium iodide. The particularly preferred composition of the foregoing ternary mixed salt is the eutectic mixture, M.P. 127 C., having a composition, in mole percent, of 52:2 sodium amide, 38:2 sodium hydroxide, and 10:2 sodium iodide. For producing a mixture of sodium and potassium (NaK) from its amalgam, a preferred molten electrolyte is a lowmelting binary mixture of sodium amide and potassium amide.

In the electrolysis of an aqueous solution of an alkali metal chloride to obtain an alkali metal utilizing an amalgam bipolar interelectrode, in accordance with the present invention, either two separate cells may be used, with the alkali metal amalgam being physically transferred between the cells, or both reactions may be carried out within a unitary electrolytic cell having two subcells interconnected through a stationary mercury bipolar electrode.

Heretofore, a unitary high-efficiency electrolytic cell for the electrolytic recovery of an alkali metal from its amalgam prior produced by the electrolysis of an aqueous chloride solution has not been feasible because of the high temperature required for amalgam electrolysis when using a high-melting molten salt electrolyte compared with the lower temperature used for the aqueous electrolysis. The present invention permits operation of a unitary cell at a temperature as low as about 135 C. under optimum conditions for sodium production and as low as about 100 C. under optimum conditions for the production of sodium-potassium mixtures, an operating temperature about 10 degrees above the melting point of the fused salt electrolyte being preferred.

Brief description of the drawings FIG. 1 is a diagrammatic representation of the cell reactions that occur in a preferred embodiment of an electrolytic cell of this invention.

FIG. 2 is a schematic representation, partly in crosssection, of a preferred embodiment of a unitary electrolytic cell of this invention.

Description of the preferred embodiments The electrolytic recovery of alkali metals from alkali metal amalgam is known and has been described in U.S. Patents 2,148,404 and 3,265,490. Substantially the same reactions occur whether a separate cell is used or whether the reaction is carried out in a subcell of a unitary electrolytic cell, as shown herein. The alkali metal is liberated at the cathode and suitably collected.

The process provided by the present invention is particularly applicable to the recovery of sodium, potassium and sodium-potassium mixtures from their molten amalgams. Where molten potassium is electrolytically recovered from its molten amalgam, a low melting molten potassium salt, e.g., a ternary mixture containing, in mole percent (m/o), 30-50 potassium amide, 20-35 potassium hydroxide, and 30-50 potassium iodide, is preferred for use as electrolyte. A particularly preferred potassium salt mixture contained 36i2 m/o KNH 27:2 m/o KOH, and 37:2 m/o KI and had a melting point of 171 C. Where a sodium-potassium mixture is to be recovered from a mixed amalgam of these metals, suitable low melting electrolytes that may be used are the NaNH -KNH binary eutectic (M.P. 93 C.), shown by C. A. Krause and E. G. Cuy in J. Am. Chem. Soc. 45, 712-715 (1923), and a mixed salt composition of the amide, hydroxide and iodide of sodium and potassium.

Of the alkali metals of interest, K, NaK and Na, the recovery of sodium is of particular interest because of its increasing industrial importance. A suitable and preferred low-melting electrolyte for the recovery of sodium is the eutectic NaNH -NaoH-NaI mixture which has a melting point of about 127 C. This electrolyte and other suitable ones are more fully described in copending application S.N. 621,577, filed Mar. 8, 1967, which should be referred to for further details. The practice of this invention will therefore be exemplified by reference to a preferred embodiment thereof, shown in FIGS. 1 and 2 of the drawings.

In the diagram of FIG. 1, there is shown an idealized representation of the reactions which occur in a dual cell system during electrolysis. In cell I a concentrated aqueous sodium chloride solution is shown as supplied continuously to an electrolyte compartment 1 through an opening 2, which is connected to a sodium chloride storage tank, not shown. Dilute sodium chloride solution is removed from compartment 1 through an opening 3. Upon the application of current, the sodium ions gain electrons at the sodium chloride-mercury interface 4 to form sodium amalgam in a compartment 5 by combination with the mercury or amalgam present in this compartment. Simultaneously, at the aqueous sodium chloride solution-anode interface 6, chlorine ions lose electrons to form chlorine gas in anode compartment 7. The chlorine gas may be removed from anode compartment 7 by way of an opening 8. Depending upon the cell operating conditions, some hydrogen may be formed by self-discharge at the interface 4 of the amalgam electrode and the aqueous sodium chloride electrolyte. This hydrogen is removed from the cell through opening 3 and collected. At the same time, in cell II, at an interface 9 between amalgam compartment 5 and a compartment 10 containing the molten salt electrolyte, the sodium in the sodium amalgam loses electrons to form sodium ions. The net result of these reactions then may be considered as the electrochemical transfer of sodium ions from compartment 1 to compartment 10 by way of amalgam compartments 5 and 5'. The sodium ions migrate through the molten electrolyte in compartment 10, and at an interface 11 between the molten electrolyte compartment 10 and cathode compartment 12 gain electrons to discharge molten sodium in the cathode compartment. This molten sodium may be conveniently removed by way of an opening 13.

The reactions which occur in cells I and II correspond to those which occur in a unitary cell containing two equivalent subcells. In the first cell (1) wherein an aqueous sodium chloride solution is electrolyzed, the amalgam serves as the cathode of this first cell. In the second cell (II) wherein sodium is produced by electrolysis of the sodium amalgam using a molten electrolyte, the amalgam serves as the anode of the cell. Thus, the amalgam in compartments 5 and 5, which circulates in countercurrent fiow thrpugh openings 14 and 14 in heat exchanger 15 acts as a bipolar electrode, serving simultaneously as the cathode of the aqueous electrolyte first cell and the anode of the molten salt electrolyte second cell. When the two cells are operated at the same temperature, they can be united to form a unitary cell having a single amalgam compartment. Heat exchanger 15 is then eliminated.

In FIG. 2 is shown a cross-sectional view of a preferred embodiment of a compact unitary electrolytic cell, in accordance with this invention, in which the two subcells of the electrolytic cell are housed together in the same unit in isothermal relation with one another, the entire electrolytic cell being operated at substantially the same temperature below C. This unitary arrangement of the subcells employing an immobilized horizontal amalgam interelectrode eliminates any requirement for circulating amalgam between two separate cells with associated conduits, pumps, and heat exchangers. Such an arrangement employing an immobilized amalgam intermediate electrode is a simpler and more efiicient way for producing sodium starting with an aqueous sodium chloride solution. However, for certain large scale units or for operation at temperatures above 135 C., it may be desirable to perform the two electrolysis steps in separate cells maintained at different temperatures.

Referring to FIG. 2, the electrolytic cell 20 may be considered as consisting of two subcells, a first aqueous subcell and a second molten electrolyte subcell, although the separate identities of the two subcells are merged under conditions of actual operation. A concentrated sodium chloride solution is fed into a cell compartment 21 by way of a conduit 22; a dilute sodium chloride solution leaves the cell compartment by way of a conduit 22a, and is conveniently stored in a storage tank therefor, not shown. A porous plastic separator 23 serves to prevent mixing of chlorine with hydrogen, since the latter may be evolved in small amounts by self-discharge at the amalgam surface of screen 28. Evolved hydrogen would leave the cell through conduit 22a. Upon electrolysis, substantially pure chlorine gas is evolved in a compartment 24 in contact with an anode electrode 25, which preferably consists of a suitable graphite electrode. Various suitable electrode configurations are known to the art and are shown, for example, in U.S. Patents 2,148,404 and 3,006,- 824. The chlorine gas may conveniently be removed by way of an opening 26 in outer wall 27 of the anode compartment, made of a suitable corrosion-resistant material, e.g., stainless steel. At the same time, sodium ions in the sodium chloride solution gain electrons and deposit at the amalgam electrode consisting of a metal screen or porous metal 28 wetted by an immobilized horizontal layer of dilute amalgam 29, contained between screen 28 and a porous ceramic matrix 30. The molten salt electrolyte is conveniently retained in the pores of matrix 30, which is preferably of a ceramic material but may also be made of a plastic material. Alternatively, the ceramic matrix layer containing the molten salt electrolyte may be replaced by a high polymer ion-exchange membrane.

Desirably, the cell is operated at as low as temperature as feasible, since the minimizes mercury loss, contamination of sodium by mercury, and permits use of less eX- pensive lightweight materials of construction that are able to withstand corrosive attack at lower temperatures. While the cell operating temperature will be kept as close as feasible to the melting temperature of the fused salt electrolyte, ordinarily the cell will be operated at a temperature about 5 to degrees above that of the melting point of the electrolyte. In order to achieve as low a cell operating temperature as feasible for the recovery of pure sodium, a cell operating temperature as low as about 130 C. may be obtained by utilizing as the low melting molten sodium salt electrolyte the eutectic composition of the ternary salt mixture NaNH -NaOH-NaI, which has a melting point of about 127 C.

The sodium ions gain electrons to form molten sodium, which is collected in cathode compartment 31 and conveniently drained therefrom by way of an opening 32 and stored in a sodium storage tank, not shown. The outer wall 33 of the sodium cathode compartment 31 is made of a suitable nonreactive conductive material, such as stainless steel, which is insulated from the amalgam interelectrode by suitable O-rings 34 made of a material such as polypropylene, hard rubber, or ceramic. Insulating O-rings are also used to electrically insulate the amalgam interelectrode from the anode compartment.

The following examples, which are illustrative only and are not to be construed as limiting the invention, describe operation of both a single amalgam cell and of a unitary electrolytic cell including both subcells.

Example 1 As illustrative of the practice of this invention utilizing two cells maintained at different temperatures, an aqueous sodium chloride solution having a concentration of 350 grams/liter is electrolyzed at a temperature of 50 C. in a first cell having a sodium amalgam cathode and a graphite anode. The two electrodes are separated by a porous plastic membrane. The cathode is a vertical steel plate down which flows a continuous stream of dilute sodium amalgam. The sodium concentration of the amalgam entering the cell is 0.5 atom percent; its concentration on leaving the cell is 3.1 atom percent. An average current density of 250 ma./cm. is used during the electrolysis. The cell voltage is 4.25 v. Chlorine is evolved at the anode.

The sodium amalgam is fed to a second electrolytic cell made of dense alumina and equipped with nickel electrode terminals. The cell is charged with a sodium salt electrolyte consisting of 52 mole percent NaNH 38 mole percent NaOH and 10 mole percent NaI. The cell is then heated to 130 C. After the electrolyte melted at 127 C., sodium amalgam containing 3.1 atom percent sodium is introduced to the bottom of the cell, followed by addition of a small amount of sodium which floats on the top of the molten electrolyte. The amalgam and sodium layers are in contact with the anode and cathode electrode terminals, respectively. The thickness of the molten salt electrolyte is 0.5 cm. A current density of 200 ma./cm. is used during the electrolysis. At the beginning of the electrolysis the operating cell voltage is about 1.2 v. At the end of the electrolysis the operating cell voltage is about 1.35 v. and the sodium content of the amalgam is about 1.0

atom percent. A coulombic efiiciency of sodium recovery of 97% can be achieved. Sodium is recoverable containing less than 0.01 wt. percent mercury.

Example 2 Sodium amalgam containing 3.1 atom percent sodium is prepared by the same method as shown in Example 1. Sodium is recovered from the amalgam in an electrolysis cell made of polypropylene, equipped with nickel electrode terminals. The composition of molten salt electrolyte is the same as in Example 1, but it is contained in the pores of an alumina disk of 45% porosity which separates the anode and cathode compartments. The thickness of the porous alumina disk is 0.2 cm. The anode compartment is filled with dilute sodium amalgam containing about 3 atom percent sodium, and the cathode compartment is filled with molten sodium. During electrolysis, the dilute sodium amalgam is continuously supplied to the anode compartment and a depleted amalgam of 0.5 atom percent sodium content is continuously removed from the anode compartment. Sodium is deposited in the cathode compartment. The electrolysis is carried out at a temperature of 130 C. At a current density of ma./cm. the operating cell voltage is 1.3 v. The coulombic efficiency of sodium recovery is greater than 98%. The sodium product is essentially free of mercury contamination.

Example 3 A unitary electrolytic cell with a structure as shown in FIG. 2 and equipped with feed and receiver storage tanks for aqueous sodium chloride solution and a third tank for sodium storage is used for the electrolysis. The cell is operated at a temperature of C. and under an inert gas pressure of 30 p.s.i.g. An aqueous sodium chloride solution of 350 grams/liter concentration is used as feed. The molten salt electrolyte has the same composition as in Example 2 and is contained in a porous alumina disk of 0.2 cm. thickness. The electrolysis is carried out at a current density of 50 ma./cm. at an operating cell voltage of 5.2 v. The chloride produced at the anode and any evolved hydrogen are vented from the cell at the cell operating pressure. Sodium essentially free of mercury contamination is collected in the sodium storage tank.

By means of the present invention, there has been provided a method for the electrolytic recovery of an alkali metal from its amalgam utilizing a molten salt electrolyte having high electrical conductivity, low melting point, and compatibility with the metal to be recovered, thereby permitting operation at temperatures substantially lower than heretofore feasible. As a consequence, loss of mercury is minimized and alkali metals are obtainable of higher purity and at considerable enhanced energy efficiency compared with methods heretofore available. The present process for certain applications also permits use of a unitary electrolytic cell having an immobilized amalgam interelectrode, the entire cell being operated at the same temperature. Also, the electrolysis in accordance with the invention may be accomplished by operating two separate cell units at different temperatures. Where two separate cell units are used, the cell in which the aqueous salt solution is electrolyzed is operated at a temperature between 20 and 100 C., Whereas the cell for the recovery of the alkali metal from its amalgam is operated at a higher temperature determined by the melting point of the molten salt electrolyte utilized. It will thus be apparent that many variations and details of cell construction, selection and concentration of alkali-metal salt, and concentration of amalgam may be practiced without departing from the scope of the present invention. Thus, while the preferred embodiments of the process of the present invention and its principle of operation have been illustrated and described, it should be understood that Within the scope of the appended claims the invention may be practiced otherwise than as specifically illustrated and described.

I claim:

1. A process for the recovery of an alkali metal selected from sodium, potassium and mixtures thereof from a molten amalgam of said selected alkali metal comprising employing said amalgam as anode in an electrolytic cell which contains as molten electrolyte a fused salt mixture having a melting point below 180 C. and containing as essential component the amide of said selected alkali metal.

2. A process according to claim 1 wherein said alkali metal is sodium and said fused salt mixture is a ternary mixed salt fusible below 160 C. and containing, in mole percent, 30-78 sodiu-m amide, 20-60 sodium hydroxide, and 1-20 sodium iodide.

3. A process according to claim 2 wherein said fused salt mixture contains, in mole percent, 52:2 sodium amide, 38:2 sodium hydroxide, and :2 sodium iodide.

4. A process according to claim 1 wherein said alkali metal is potassium and said fused salt mixture has a composition, in mole percent, of 30-50 potassium amide, 210-35 potassium hydroxide, and 30-50 potassium iodide.

5. A process according to claim 4 wherein said fused salt mixture contains, in mole percent, 36:2 potassium amide, 27:2 potassium hydroxide and 37:2 potassium iodide.

6. A process according to claim 1 wherein said alkali metal consists of a mixture of sodium and potassium and the molten electrolyte is a binary mixture of sodium amide and potassium amide.

7. A process for the production of sodium from an aqueous solution of sodium chloride utilizing a unitary electrolytic cell containing first and second subcells, the electrolytic cell being operated at a temperature below 135 0, comprising (a) maintaining in said first subcell an aqueous solution of sodium chloride in contact with an anode and an amalgam of sodium as cathode,

(b) maintaining in said second subcell a fused salt mixture molten at cell operating temperature and containing, in mole percent 52:2 sodium amide,

38:2 sodium hydroxide, and 10:2 sodium iodide as molten electrolyte in contact with a sodium amalgam anode and a cathode,

the cathodic amalgam of said first subcell serving as anodic amalgam of said second subcell, and

(c) passing an electric current through the cell by electrically contacting the anode of said first subcell and the cathode of said second subcell whereby sodium is produced at the cathode of said second subcell.

8. A process for the production of sodium-potassium metal from an aqueous solution of sodium and potassium chlorides utilizing a unitary electrolytic cell containing first and second subcells, the electrolytic cell being operated a a temperature below C., comprising (a) maintaining in said first subcell an aqueous solution of sodium and potassium chlorides in contact with an anode and a sodium-potassium amalgam as cathode,

(b) maintaining in said second subcell a fused salt mixture molten at cell operating temperature and containing, a binary mixture of sodium amide and potassium amide as molten electrolyte in contact with a sodium-potassium amalgam anode and a cathode,

the cathodic amalgam of said first subcell serving as anodic amalgam of said second subcell, and

(c) passing an electric current through the cell by electrically contacting the anode of said first subcell and the cathode of said second subcell whereby sodium-potassium metal is produced at the cathode of said second subcell.

References Cited UNITED STATES PATENTS 3/1937 Wait 20468 XR 3/1939 Moltkehansen 204-68 Disclaimer 3,472,746.Las2lo A. Herdy, Canoga Park, Calif. ELECTROLYTIC SYS- TEM FOR PRODUCTION OF ALKALI METALS. Patent; dated Oct. 14, 1969. Disclaimer filed Nov. 2, 1970, by the assignee, Nomi/z American Rockwell Gowpomtz'on. Hereby enters this disclaimer to claims 1, 2, 6 and 8 of said patent.

[Ofiiczal Gazette March 2, 1.971.] 

